The invention relates to a method for operating an exhaust gas purification system of a motor vehicle internal combustion engine, the system having a selective catalytic reduction (SCR) catalyst for the selective catalytic reduction of nitrogen oxides.
A method for operating an exhaust gas purification system having an SCR catalyst for the selective catalytic reduction of nitrogen oxide (NOx) is known from DE 10 2008 036 884 A1, wherein a metering rate for a reducing agent containing ammonia for NOx reduction is adapted by changing an adaptation factor if appropriate, if there are excessive deviations from emission values obtained by modeling and by measurement. In this case, NOx emission values are captured metrologically by means of exhaust gas sensors both before and also after the SCR catalyst.
The object of the invention is to provide a method which is simplified in this connection.
In the method according to the invention a reducing agent containing ammonia (NH3), in particular an aqueous urea solution, is metered into the exhaust gas at a predeterminable metering rate resulting from a base value and an adaptation factor correcting the base value. From time to time, a test procedure is carried out in order to check and, if appropriate, to adjust the adaptation factor. In this test procedure, a first nitrogen oxide value of the exhaust gas is compared with a second nitrogen oxide value of the exhaust gas, wherein the first nitrogen oxide value and the second nitrogen oxide value are determined by one and the same nitrogen oxide sensor disposed downstream of the SCR catalyst. To this end, the first nitrogen oxide value is determined when the metering of the reducing agent is deactivated and the second nitrogen oxide value is determined when the reducing agent is metered at a test metering rate which can be predetermined.
The checking of the adaptation factor which is carried out from time to time makes it possible to recognize and, if appropriate, to correct changes occurring due to aging or drift phenomena in the exhaust gas system. If the changes exceed a predeterminable extent, provision is made to carry out an adjustment of the adaptation factor and thus an adaptation of the metering rate to the changed conditions. This enables compensation for example for drift phenomena from system components and inaccuracies which are contingent upon aging, so that a high nitrogen oxide purification rate can also be achieved over relatively long operating periods. The metering rate is preferably produced from the base value and the adaptation factor by multiplicative combination. To this end, the base value is preferably determined by a computer model or is inferred from characteristic fields. With the same predetermined operating conditions, the same base values are preferably used. Time-related system changes which necessitate an adaptation of the metering rate in order to enable high nitrogen oxide reduction values to be achieved over a long period of time, are preferably compensated for by an adjustment of the adaptation factor.
Because one and the same nitrogen oxide sensor is used for capturing the first and the second nitrogen oxide value, it is possible to omit a second nitrogen oxide sensor used upstream of the SCR catalyst, resulting in corresponding cost advantages for the exhaust gas purification system according to the invention. An upstream NOx sensor is used frequently in exhaust gas purification systems which are known from the prior art for the determination of the raw NOx emission, i.e., for the determination of the amount of NOx flowing into the SCR catalyst with the exhaust gas. In the present case, the downstream NOx sensor is used for this purpose. In this case advantage is taken of the knowledge that, with the metering of the reducing agent deactivated, the SCR catalyst is ineffective with regard to its nitrogen oxide reduction function, and for this reason the NOx concentration in the exhaust gas, measured at the discharge end of the SCR catalyst by means of the preferably single NOx sensor present in the exhaust gas purification system, corresponds to the NOx concentration in the exhaust gas present at the intake end. This is equal to the raw NOx emission from the internal combustion engine in the absence of further exhaust gas purification components which can effect an NOx reduction. If the reducing agent is then metered in at a predeterminable test metering rate, an NOx reduction of the SCR catalyst corresponding to the test metering rate can be determined from the NOx emission measured downstream of the SCR catalyst using the raw NOx emission previously determined with the metering deactivated. If this deviates from the base value modeled for the corresponding operating point, the adaptation factor is adapted in such a way that the previously determined NOx reduction is produced by computation with the adjusted adaptation factor. Until a further check and, if appropriate, repeated adjustment of the adaptation factor is performed at a later time, metering of the reducing agent then takes place at a metering rate which is determined by the base value and the adjusted adaptation factor.
In one embodiment of the invention, in an SCR catalyst with the capability for storing ammonia, the first nitrogen oxide value is determined with an SCR catalyst free of stored ammonia. In this way it is ensured that the first nitrogen oxide value actually corresponds to the raw NOx emission of the internal combustion engine and it is not for instance the case that, due to residual quantities of ammonia stored in the SCR catalyst, which would effect a reduction of NOx in the SCR catalyst, a nitrogen oxide value which deviates from the raw NOx emission is measured. In order to free the SCR catalyst from stored reducing agent, provision is preferably made to operate the SCR catalyst with the metering deactivated for a predeterminable time period and, only after this time period has elapsed, to determine the first nitrogen oxide value.
In a further embodiment of the invention the test procedure is carried out following a thermal regeneration of a particle filter associated with the exhaust gas purification system. Since a thermal regeneration of the particle filter takes place at an elevated exhaust gas temperature of approximately 550° C. or more, the SCR catalyst has, following the particle filter regeneration, an elevated temperature and therefore in any case a small amount of stored ammonia. Therefore, the operation of freeing the SCR catalyst of stored ammonia when carrying out the test procedure may be brief or may even be completely omitted.
In a further embodiment of the invention, the base value is determined by a computer model which uses an estimated operating point-dependent nitrogen oxide conversion capacity of the SCR catalyst and an estimated operating point-dependent raw NOx emission of the internal combustion engine. In this case, for estimating the operating point-dependent nitrogen oxide conversion capacity of the SCR catalyst, exhaust gas operating parameters which influence the conversion capacity of the SCR catalyst, such as exhaust gas temperature, exhaust gas throughput and emission values, are preferably determined by computation and/or by sensors and a corresponding conversion capacity is determined by computation or is inferred from previously stored characteristic curves. The raw NOx emission of the internal combustion engine is preferably determined with reference to a computed raw NOx emission model and from previously stored characteristic curves as a function of engine load and engine speed as well as, if appropriate, further influencing factors. The use of a nitrogen oxide sensor disposed upstream from the SCR catalyst is preferably omitted.
In a further embodiment of the invention, when the second nitrogen oxide value is determined, a test metering rate is set which corresponds to a predeterminable desired NOx conversion. The test metering rate is preferably set so that an NOx reduction of the SCR catalyst which is below the maximum possible value is produced by computation. This avoids an unintentional over-metering, which could lead to an ammonia slip and an incorrect measurement of the second nitrogen oxide value. In particular in a further embodiment of the method according to the invention, it is provided that a test metering rate is set which corresponds to a computed desired NOx conversion of the SCR catalyst in a range from 20% to 80%, particularly preferably in a range from 40% to 70%.
In a further embodiment of the invention, the test procedure is carried out under stationary or quasi-stationary conditions. These should be understood to be conditions under which substantially constant values or at most slight fluctuations of values for exhaust gas temperature and raw NOx emission are present. As a result it is ensured that a conversion rate of the SCR catalyst calculated from the first and the second nitrogen oxide value can be unambiguously associated with a specific operational state. If the operating conditions change during the test procedure beyond a predeterminable extent, provision is preferably made to cancel the test procedure and to restart it at a later time.
In a further embodiment of the invention it is provided that the first and/or the second nitrogen oxide value is/are obtained by integration of a sensor signal provided by the nitrogen oxide sensor. As a result, an improved precision and reliability in the determination of the first and the second nitrogen oxide value are achieved. The integration of the values preferably takes place over a predeterminable time period.
Further advantages, features and details of the invention are disclosed by the following description of preferred embodiments and with reference to the drawings. The features and combinations of features stated above in the description and the features and combinations of features stated below in the description of the drawings and/or in the drawings alone can be used not only in the combination specified in each case, but also in other combinations or in isolation, without departing from the scope of the invention.